Everyday motion creates considerable wear and tear on orthopedic joints. This is especially true for knee joints, which support a patient's weight. In addition, some diseases like arthritis accelerate the degeneration of joints. Damage to the joint tissues causes pain and loss of joint function. To repair damaged joints, surgeons can replace the entire joint with an artificial joint replacement. Over 500,000 patients have knee replacement surgery each year. Total knee arthroplasty surgery typically involves affixing a femoral component to the end of a patient's femur, affixing a tibial component to the end of a patient's tibia, and inserting a tibial insert between the tibial component and the femoral component. A tibial insert has a base that contacts the tibial component and a posterior stabilizing post that contacts the femoral component. The tibial insert moves like a hinge relative to the femoral component, and the hinge-like motion allows the knee to flex. The human knee and many knee replacements display rollback at high flexion. Rollback occurs when the femur's contact point with the tibia moves towards the posterior of the tibia.
A replacement joint may be customized relative to the patient's anatomy. Preferred replacement joints allow the patient an optimal degree of rollback. Some rollback is often desirable to mimic the behavior of the natural knee, but excessive rollback may be detrimental because it over-stretches the soft tissues surrounding the joint (e.g., the patellar tendon and quadriceps). Over-stretching of these tissues can cause the tissues to tighten, ultimately restricting the patient's movement. Because soft joint tissue structures vary from patient to patient, no single joint replacement may be appropriate for all patients.
Current replacement implants underperform relative to natural joints in several aspects. First, current implants do not allow a user to customize the geometry and motion of the insert's posterior stabilizing post. Second, replacement joints sometimes offer insufficient support over the joint's range of motion because a patient does not have the requisite tendon strength to hold the joint in position; the amount of support necessary varies from patient to patient. Third, flexion and rollback of a replacement joint can feel unnatural to a patient. Because joint implants are often made of metal and rigid plastic, flexion and rollback can come to a sudden halt instead of gradually slowing. There is a need in the art for a replacement joint with a customized posterior stabilizing post that supports the joint and/or cushions flexion or rollback at the edge of the joint's range of motion.